An experiment was carried out in order to study the effects of thermal treatment and of the addition of dolomitic limestone on the release of potassium from a nephelinic sienite from Poços de Caldas. The potassium silicate was blended with dolomitic limestone in the proportions, respectively, of 1:1 and 3:1. The mistures were calcined at the temperatures of 1000, 1100, 1200 and 1300°C and subject to fusion. The materials afterwards were shaken with water (ratio 1:100, w/v) for 1, 4, 8, 12 , 24, 36 and 48 hours. The untreated K rock was used as control. Both thermal treatment and addition of dolomitic limestone, increased the liberation of K which was higher when the mixture was prepared using the 1:1 ratio and the temperatures of calcination were in the range of 1000 - 1100°C.